The present invention relates to a measuring instrument, by which it is possible to measure a distance to an object to be measured by projecting a distance measuring light to the object to be measured and by receiving a reflection light from the object to be measured.
As the method to measure a distance to an object to be measured by an electronic distance measuring instrument, there are two methods. One method is a method of measurement to use a prism, in which a prism is installed at a measuring position, and a distance measuring light reflected by the prism is received by using the prism as an object, and the distance is measured. Another method is a method of measurement on non-prism mode not to use a prism, in which a distance measuring light is directly projected to the object to be measured and the distance is measured by receiving a reflection light from the object to be measured.
In case the measurement is performed by using the prism, reflectivity of the prism is high, and a reflection light with high light amount is obtained. Thus, the measurement can be performed on a long distance. In contrast, in case of the non-prism mode, there is no need to install the prism each time at the measuring position, and distance measurement can be performed simply, but the reflected light amount is lower, and it is not possible to measure a long distance. Therefore, more preferably, it should be so designed that the measurement mode can be selected adequately to suit the situation of the measurement.
Next, in the measurement mode using the prism, as shown in FIG. 4, in case a distance measuring optical axis 1 is deviated from a prism optical axis 3 of a prism 2, or in case the distance measuring optical axis 1 is deviated from a center of the prism 2, a distance measuring light reflected by the prism 2 (hereinafter referred as “reflected distance measuring light”) is deviated from the distance measuring optical axis 1. For this reason, at a photodetecting unit 4 for receiving the reflected distance measuring light, the light receiving position is deviated by an amount of “d” from a standard position (i.e. a position, which corresponds to the distance measuring optical axis).
When it is seen from the viewpoint of manufacture, the photodetecting unit 4 does not necessarily have uniform sensitivity or responsiveness over total surface of the light receiving surface, and the sensitivity or the responsiveness differs according to each point on the light receiving surface. For this reason, when the distance measuring optical axis 1 is deviated from the prism optical axis 3, there is a case that the responsiveness of a photodetection signal from the photodetecting unit 4 may differ according to the light receiving position of the light receiving surface 4a. When the responsiveness of the photodetection signal differs, the difference of responsiveness appears as an error in the value of the measured distance when the distance is measured.
Therefore, the deviation of the distance measuring optical axis 1 from the prism optical axis 3 becomes a factor of an error in the measurement. Meanwhile, as cases where the prism optical axis 3 is deviated from the distance measuring optical axis 1, there are a case where the distance measuring instrument has automatic sighting function and sighting error is caused when sighting is performed sequentially, a case where there is a heat haze in an atmosphere when a long distance is measured, a case where there is sighting error caused by a surveying operator, and other cases. On the other hand, there may be a request from a customer side that, even when the distance measuring optical axis 1 is deviated from the prism optical axis 3, the distance measurement accuracy must be at a predetermined value or lower until an amount of the deviation between the distance measuring optical axis 1 and the prism optical axis 3 reaches a predetermined value.
In the past, when there has been such a request from the customer side, it has been practiced that a photodetecting unit 4 of high quality, having the light receiving surface 4a, was selected, and even when there was a deviation between the distance measuring optical axis 1 and the prism optical axis 3, a predetermined measurement accuracy should have been maintained. Therefore, the yield was low on the photodetecting unit 4, and higher cost is required.
In the past, in the distance measuring instrument, which is capable of the prism measurement and the non-prism measurement, the amount of the reflection light from the object to be measured may differ extensively from each other. Therefore, it has been practiced to provide a light amount adjuster on the distance measuring light projecting optical system and to adjust the light amount of the distance measurement.